Method for removing colloids from lithographic printing surfaces



. Patented Nov. 24, 1936 METHOD FOR REMOVING COLLOIDS FROM LITHOGRAPHIC PRINTING SURFACES Robert F. Reed, Paul W. Dorst, and Anthony George, Cincinnati, Ohio, assignors to The Lithographic Technical Foundation, Inc., New York, N. Y., a corporation of Delaware No Drawing. Application November 28, 1933,

I Serial No. 700,115

6 Claims.

This invention relates to a process for removing light-hardened bichromated colloid coatings and the like from lithographic plates.

In all processes for preparing reversed or deepetched (intaglio) lithographic plates it is necessary to produce a resist or stencil to protect the non-printing areas while the design areas are being etched. This resist or stencil must be removed before the plate is ready to print.

Usually the stencil consists, either wholly or in part, of light-hardened bichromated colloid,

means of alkali metal hydroxides possesses th following disadvantages:

1. These alkalies are extremely active and even 5 dilute solutions cause irritation or corrosion of the skin of operators.

2. Before removal of the resist, the design on the plate or stone must be treated with an ink, varnish or lacquer to form a ground capable 30 of receiving ink during the printing operation, which must be resistant to the caustic alkali used for removal of the resist. Inks containing fatty oils are unsuitable for this ground as they are attacked and removed from the design areas by the caustic alkalies. Tinctures containing asphaltum, certain synthetic resins, or pyroxylin, which are relatively alkaliresistant, are necessary but these possess the disadvantage that the plate, after treatment with caustic alkali, often 40 fails to accept ink, or accepts ink only with dif- Removal of the etching resist or stencil with an acid possesses the following disadvantages:

1. Dilute mineral acids rapidly attack the n grain of the lithographic surface in the nonprinting areas and damage it by reducing its tooth or water-carrying capacity in proportion to their activity and duration of contact.

2. Inks containing fatty ofls and pigments are 50 easily penetrated by dilute mineral acids which then attack the metal or stone underneath.

Grounds prepared with asphaltum, pyroxylin or synthetic resin tinctures, which are satisfactory acid resists, often give trouble by failing to ac- 55 cept ink.

As an improvement in the process of preparing reversed or deep-etched plates we have found that etching resists composed of or containing lighthardened bichromated glue may be removed by solutions containing an alkaline earth hydroxide, a salt of an alkaline earth, or a mixture of an alkaline earth hydroxide and a salt of an alkaline earth.- Such solutions possess the following advantages over alkali metal hydroxides.

'1. They are not as corrosive to the skin of oper ators as solutions of alkali metal hydroxides.

2. They do not attack and remove from the design areas a ground prepared with inks composed of fatty oils and pigments. It is to be understood that the term lithographic printing surface as used herein refers to all surfaces used in lithographic printing with the exception of metals such as aluminum which are easily attacked by alkalies.

Examples of solutions suitable for removal of lighthardened bichromated glue resist or resists containing glue in admixture with another colloid or colloids:

Examples 2 and 4 show how a composition covered by this application may be prepared by forming the alkaline earth hydroxide in solution by the action of an alkali metal hydroxide on a salt of an alkaline earth chloride. The presence of some alkali metal chloride and hydroxide due to 1. Calcium chloride grams 5 Calcium hydroxide do 2 Water to make cc 100 2. Calcium chlorides grams 8 Sodium hydroxide gram 1 Water to make cc 100 3. Barium chloride grams 5 "Barium hydroxide do 2 Water to make 00 100 4. Barium chloride grams 8 Sodium hydroxide gram 1 'Water to make r 100 5. Barium hydroxide grams 2 Water to make cc 100 the existence of a chemical equilibrium does not I interfere with the proper working of the solution.

The limits of concentration found to be most eifective are as follows: Alkaline earth hydroxide 0.1gram/100 cc.

to saturation Alkaline earth an-.- 1.0 to 15 grams 100 cc. While alkaline earth chlorides are referred to in the examples other alkaline earth salts may be employed with equally good results.

Having described our invention, we desire to be limited only by the following claims:

1. A new method of removing bichromated colloid films containing glue from lithographic printing surfaces, said method, comprising, treating the lithographic surfaces with a solution of an alkaline earth hydroxide in water.

2. A new method of removing bichromated colloid films containing glue from lithographic printing surfaces, said method, comprising, treating the lithographic surfaces with a solution of an alkaline earth hydroxide and a salt of an alkaline earth in water.

3. A new method of removing bichromated colloid films containing glue from lithographic printing surfaces, said method, comprising, treating the lithographic surfaces with an aqueous solution containing an alkaline earth chloride and an alkaline earth hydroxide.

4. A new method of removing bichromated colloid films containing glue from lithographic printing surfaces, said method, comprising, treating the lithographic surfaces with a solution of an alkaline earth hydroxide in water, the concentration of the hydroxide being from 0.1 gram per 100 cc. to saturation.

5. A new method of removing bichromated colloid films containing glue from lithographic printing surfaces, said method, comprising, treating the lithographic surfaces with a solution of an alkaline earth hydroxide and a salt of an alkaline earth in water, the concentration of the alkaline earth hydroxide being from 0.1 gram per 100 cc. to saturation, and the concentration of the alkaline earth salt being from 1.0 to 15 grams per 100 cc.

6. The method of removing bichromated colloid films from lithographic. printing surfaces which comprises, treating the surfaces with an aqueous solution containing a material selected from the group consisting of an alkaline earth hydroxide, salts of alkaline earth, and mixtures of alkaline earth hydroxide and alkaline earth salts.

ROBERT F. REED. PAUL W. DORST. ANTHONY GEORGE. 

